Hydrogen-containing gas is often used as an anode gas which is the fuel gas in power generation by a fuel cell power generation system, and it is generally the case that a fuel processing apparatus for generating hydrogen-containing gas by using a steam reforming technique is installed together with the fuel cell.
A fuel processing apparatus which utilizes the steam reforming technique includes a reformer for generating hydrogen-containing gas by causing a steam reforming reaction and a heating section for supplying the heat which is necessary for that steam reforming reaction. The reformer contains a reforming catalyst, e.g., a precious metal type catalyst such as platinum, ruthenium, or rhodium, or a Ni type catalyst. To this reformer, a hydrocarbon-type raw material such as natural gas, LPG, naphtha, gasoline, or kerosene, or an alcohol-type raw material such as methanol, and water are supplied. Then, when the reformer is heated by the heating section to a temperature which is suitable for the steam reforming reaction, hydrogen-containing gas is generated by the action of the reforming catalyst.
In this steam reforming reaction, about 10 to 15% (dry gas basis) of carbon monoxide (hereinafter denoted as CO) is generated as a secondary component. CO poisons the catalyst which is used for the electrodes of the fuel cell, thus lowering the power generation ability. Therefore, in the fuel processing apparatus, a shift converter and/or a CO remover are also installed in order to reduce the CO concentration in the hydrogen-containing gas. The shift converter contains a shift catalyst for allowing CO to react with steam, thus causing a shift into hydrogen and carbon dioxide. As the shift catalyst, a precious metal type catalyst such as platinum, ruthenium, and rhodium, a Cu—Zn type catalyst, an Fe—Cr type catalyst, or the like is used, for example. Moreover, the shift converter is controlled to a temperature which is suitable for the shift reaction (shift reaction temperature), and in many cases, reduces the CO concentration in the reforming gas to about 0.5% or less.
On the other hand, the CO remover contains a selective oxidation catalyst for oxidizing the CO within the reforming gas. As the selective oxidation catalyst, a precious metal type catalyst such as platinum, ruthenium, and rhodium, or the like is used. The CO remover causes an oxidation reaction of CO by using the air which is supplied to its interior, thus lowering the CO concentration within the reforming gas to 100 ppm, and preferably to 10 ppm or less.
In order to use a fuel cell power generation system in a general household and obtain a high energy efficiently, it is desirable to boot or stop the fuel cell power generation system in accordance with the electric power load and heat load in the household. However, in order to generate hydrogen-containing gas, it is necessary to warm the reformer, the shift converter, and the CO remover to an appropriate temperature (this being referred to as a boot of the fuel processing apparatus), and thus the boot of the fuel processing apparatus requires a certain amount of time. At the boot of the fuel processing apparatus, first, the reforming catalyst in the reformer is heated by the heating section. At this time, the raw material and water are allowed to pass through the reforming catalyst, thus conveying the heat of the heating section to the shift converter to also increase the temperature of the shift catalyst. At this time, a method of directly heating the shift catalyst by using a heating structure such as an electric heater may be adopted. However, from the standpoint of reducing the number of parts and realizing a low cost, a method of heating the shift catalyst by supplying the raw material and water from the reforming catalyst to the shift catalyst, without installing an electric heater, is useful.
However, if water is supplied when the temperature of the shift catalyst is low, the water may condense on the shift catalyst. For example, in the case where a Cu—Zn type catalyst is used as the shift catalyst, the condensed water oxidizes and deteriorates the catalyst, and therefore a method of supplying water after a sufficient increase in the temperature of the shift catalyst is generally adopted.
Moreover, in order to prevent steam condensation and oxidation of the reforming catalyst, Patent Document 1 proposes a method of monitoring the temperature of the reforming catalyst, and supplying a gas which is selected from among air, combustion exhaust, steam, raw material gas, and nitrogen in accordance with the temperature of the reforming catalyst.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-93447